Acid-Base Balance and Body Temperature Regulation of Heat Stressed Psammomys obesus (Gerbillinae): The Effect of Bicarbonate Loss via Saliva Spreading

Abstract

Psammomys obesus, a diurnal desert rodent, was exposed to air temperatures (Ta; Tb = body temperature) of 24–40 C and relative humidity (RH) > 30%. Between 24 and 38 C Ta, a positive Tb − Ta gradient for effective dry heat dissipation exists. At 40 C Ta, hyperthermia develops and is accompanied by intensive spreading of saliva essential for body cooling. The arterial blood acid-base status, determined in chronically cannulated animals, is kept normal (pH = 7.42; PCO₂ = 30.1 mm Hg) at 28-38 C Ta. During 2 h at 40 C, metabolic acidosis develops and is partially compensated for by respiratory alkalosis (PCO₂ = 21.2 mm Hg), resulting in pH 7.32. After 2 h of salivation (animals were treated with pilocarpine to increase saliva flow), the heat stressed animals' saliva remains very alkaline (standard bicarbonate [HCO₃⁻] ST = 42.1 meq/liter, pH = 8.42), but extreme [HCO₃⁻] depletion develops in the blood ([HCO₃⁻] = 10. 1 meq/liter). These data strongly suggest the existence of a mechanism having negative value for maintenance of [H +]. It enhances bicarbonate drainage from blood through long-term salivation. In gerbils exposed to extreme thermal stress, saliva spreading is an essential cooling mechanism for Tb regulation. This may result in severe acidosis, which may limit the ability to maintain normal homeostasis during exposure to heat stress.